The invention relates to a process for the preparation of granules by spraying a liquid material in a fluidized bed of solid nuclei, upon which the nuclei grow by solidification of the liquid material on them, and removing the granules thus formed from the fluidized bed.
A similar process is known from The Soviet Chemical Industry 4 (1972) No. 7, pp. 456-458, and 5 (1973), No. 4, pp. 265-267, and from Verfahrenstechnik 9 (1975) No. 2, pp. 59-64.
In these known processes, the liquid material, for example in the form of a solution, melt or suspension, is with the aid of a gas sprayed to droplets, which on the fluidized nuclei solidify to form granules of the desired size. In order for the granulation process to proceed well, it is necessary that the surface of the grown nuclei solidifies sufficiently quickly and that any water present evaporates quickly, to prevent agglomeration of individual particles. In the known processes this is achieved by spraying the liquid material to fine droplets, or even nebulizing it. Of course, the rule here is that the more water is to be evaporated, the finer the nebulization should be. The size of the droplets obtained in spraying is mainly determined by the pressure and the quantity of the spraying gas, the general rule being that as this pressure and quantity are higher, the drops obtained are smaller. It is therefore common practice to apply a fairly high feed pressure, for example 1.5 bar or more as described in, inter alia, Khim. Naft. Mashinostr. (1970) No. 9, pp. 41-42, and in U.S. Pat. No. 4,219,589, to obtain droplets of relatively small average diameter.
A disadvantage of these known processes is that for spraying the liquid material to droplets a large amount of gas of high pressure is required, which, of course, is accompanied by a high energy consumption. A possible explanation for this is that the liquid material contacts the gas as a jet. The jet is broken up into drops by the gas stream pealing the outer layers from the jet.
This process continues along some distance in the downstream direction. In this process, the gas is necessarily slowed down, resulting in progressively poorer nebulization. In order still to achieve a sufficiently fine nebulization, therefore, a high mass ratio of highenergetic gas to liquid material must be applied. It has been found that for sufficiently fine nebulization of any liquid material this ratio should generally be higher than 1. Admittedly, it is possible to conduct this known spraying method with a lower mass ratio of gas to liquid material, but then a gas stream of very high feed pressure must be applied, for example more than 4 bar. The disadvantage of this is that it is accompanied by an even higher energy consumption.
In principle, fine nebulization can be achieved also by hydraulically spraying the liquid material, at very high liquid feed pressure (tens of bars). The energy consumption is then lower than in the previously described processes, but this method has the disadvantage of extreme wear on the spraying device. Moreover, serious agglomeration of nuclei in the fluidized bed is found to occur with this spraying method.
According to another known process, which is described in, for example, GB Nos. A 2,019,302 and 2,075,908, in a fluidized-bed granulation process the liquid material is with the aid of a hydraulic sprayer divided into relatively large drops, which are subsequently finely nebulized with the aid of a powerful gas stream. To this end, the liquid material is sprayed upward in the fluidized bed of nuclei via a sprayer provided with two concentric channels, the liquid material being supplied through the inner channel and the drops being contacted, shortly after leaving this channel, with a powerful gas stream supplied through the outer channel. By the powerful gas stream above the sprayer, a zone is created in the fluidized bed with a very low concentration of nuclei, the so-called dilute zone, into which nuclei are aspirated from the fluidized bed to be moistened with droplets of liquid material. Although in this known process the required amount of high-energetic gas is lower than in the processes mentioned in the introduction, this amount is found to be still quite substantial. It has been found that for good nebulization of all liquid material the required mass quantity of high-energetic gas must be more than 50% of the mass quantity of liquid material.